1. Field of the Invention
It is possible to use both pressure-controlled and stroke-controlled injection systems to supply fuel to combustion chambers of autoignition internal combustion engines. In addition to unit fuel injectors, these fuel injection systems are also embodied in the form of unit pumps and accumulator injection systems. Accumulator injection systems (common rails) advantageously permit the injection pressure to be adapted to the load and engine speed. It is generally necessary to achieve the highest injection pressure possible in order to achieve high specific loads and reduce engine emissions.
2. Prior Art
The achievable pressure level in accumulator injection systems in use today is currently limited to approximately 1600 bar for strength reasons. In order to further increase pressure in accumulator injection systems, these common rail systems make use of pressure boosters.
EP 0 562 046 B1 has disclosed an actuation/valve apparatus with damping for an electronically controlled injection unit. The apparatus has an electrically excitable electromagnet with a fixed stator and a movable armature. The armature has a first and second surface. which define a first and second cavity, the first surface of the armature pointing toward the stator. A valve is connected to the armature in a position to convey a hydraulic actuating fluid to the injection apparatus from a sump. A damping fluid can be collected in or released from one of the cavities of the electromagnet apparatus. A region of the valve that protrudes into a central bore can selectively open or close the flow connection of the damping fluid in proportion to its viscosity.
DE 101 23 910.6 relates to a fuel injection apparatus used in an internal combustion engine whose combustion chambers are supplied with fuel via fuel injectors which are acted on by means of a high-pressure source; in addition, the fuel injection apparatus also includes a pressure booster that has a movable pressure booster piston, which divides a chamber that can be connected to the high-pressure source from a high-pressure chamber that is connected to the fuel injector. The fuel pressure in high-pressure chamber can be varied by filling a rear chamber of the pressure booster with fuel or by emptying the fuel from this rear chamber.
The fuel injector has a movable closing piston for opening and closing the injection openings oriented toward the combustion chamber. The closing piston protrudes into a closing pressure chamber so that it can be acted on by the pressure of the fuel. As a result, a force is exerted on the closing piston in the closing direction. The closing pressure chamber and an additional chamber are comprised of a common working chamber; all of the subregions of the working chamber are permanently connected to one another to permit the exchange of fuel.
With this design, by triggering the pressure booster via the rear chamber, it is possible to keep triggering losses in the high-pressure fuel system low in comparison to a triggering by means of a working chamber that is intermittently connected to the high-pressure fuel source. In addition, the high-pressure chamber is pressure-relieved only down to the pressure level of the high-pressure accumulator and not down to the leakage pressure level. On the one hand, this improves the hydraulic efficiency and on the other hand it allows a quicker increase of pressure up to the system pressure level, thus making it possible to shorten time intervals between injection phases.
In pressure-controlled common rail injection systems with pressure boosters, the problem arises that it is not possible to assure the stability of the injection quantities to be injected into the combustion chamber, particularly when producing very small injection quantities, for example during preinjection. This is primarily due to the fact that the nozzle needle opens very quickly in pressure-controlled injection systems. As a result, very small variations in the triggering duration of the control valve can have a powerful impact on the injection quantity. Attempts have been made to remedy this problem by using a separate needle stroke damper piston that delimits a damping chamber and must be guided in a clearance fit that is impervious to high pressure. Although this design does in fact permit a reduction in the needle opening speed, it increases the structural complexity and therefore the cost of the injection system quite considerably.
In view of the ever-stricter standards regarding emissions and noise production of autoignition internal combustion engines, further steps must be taken in the injection system in order to meet the even tighter emissions standards to be expected in the near future.